Is there any play/slop? Try to lock out your fork, and shake your handlebars back and forth, side to side (while keeping the bars straight). There should be no movement in the steer tube. If there is, it just may need to be tightened.. or, it could be a sign of something worse.

Why are you asking? It may be easier to describe your concern, other than ask for all the possible signs of a failing headset.

Just wondering how long a typical headset is supposed to last and how do you know if it's going bad?

Grab the front brake, put your thumb and forefinger around the cup/race so that you can feel both simultaneously and rock the bike back and forth. If you feel any play its loose. Rotate the bars with the fron wheel off the ground, there should be no binding or indexing.

Now test ride it. If you can ride a straight line with no hands, if the front end does not chatter when you hit the brakes, headset is good. A properly adjusted headset willl last years.